1. Field of the Invention
This invention relates to electronic linear tape measures or rules, and more particularly to an electronic tape measure which provides an easily read digital display of the length measured by the tape alone or the tape plus the length of a side of the body of the measuring tape container or body in the instance where an internal measurement is to be determined.
2. Prior Art
Electronic tape measures have been available for a number of years. In many devices the length measured or read, has been determined by first mechanically tracking the length of tape unwound from the storage reel within the measure body by counting mechanical links in the tape with a sprocket wheel and converting to electronic pulses. The sprocket wheel drives a shaft which in turn has a wheel at its opposite end with optic, magnetic or some other indicia thereon for reading by an appropriate sensor and conversion to electronic pulses ultimately for display, memory storage, or use in calculations. Such measuring rules tend to be unreliable and not as accurate as they might be because of the multiple mechanical links between the tape and the electronic sensor and calculator portions of the device. Each such link is susceptible to breakage, play, and therefore inherent inaccuracy of the reading displayed, stored, or utilized in calculations results. An example of this type of device is disclosed in U.S. Pat. No. 4,186,490, Quenot.
An involved mechanical linkage is also utilized in U.S. Pat. No. 4,031,360, Soule wherein the tape is fabricated of joined individual links which mesh with teeth on a sproket wheel, which in turn is connected to a shaft, which at the other end has a sprocket gear that as the shaft turns in relation to movement of the linked "tape", mechanically actuates a switch to, in turn, generate pulses for the electronic portion of the device.
In U.S. Pat. No. 4,185,390, Tateishi a photosensor is used and has inherent problems in functioning such as providing sufficient light and the fact that visible indicia on the tape are often partially obscured by dirt picked up in use.
U.S. Pat. No. 4,316,081, Washizuka et al, senses the passage of spaced magnetic balls imbedded in a "tape." The balls are mounted on a thin thread such as would be done in making a pearl necklace. There are thus no visible indicia available on the "tape" for direct reading which is of course a great disadvantage.